Welcome to the forum. Thanks for your question and your confidence in our forum. Congratulations on the responsibility for sexual safety implied by these plans. There's an easy answer to your dilemma.

Until reading this, I was unaware of the problem with phenytoin and dolutegravir, one of the two drugs in Truvada and its generic versions (the other is called emtricitabine) -- and apparently your ID doc also wasn't aware. Phenytoin speeds up metabolism of dolutegravir, reducing its blood levels. Whether it would remain effective as PrEP hasn't been studied; my best judgment is that it probably would still be effective as PrEP, but you don't need to take the chance. Ask your LGBT center (or perhaps the ID specialist) to prescribe Descovy instead of Truvada. Descovy is emtricitabine plus tenofovir, which has no interaction with phenytoin -- you can take them together with complete confidence in PrEP effectiveness. The main problem might be cost, depending on your insurance coverage:  unlike Truvada, there is no generic version of Descovy. That said, it should be possible to prescribe tenofovir and emtricitabine separately; discuss with your clinic or doctor. (FYI, if you google "drug interactions", you'll find several websites where you can enter any combination of two or more drugs to see if there is any interference between them.)

Final comment:  PrEP is highly effective to prevent HIV and doxy-PEP prevents syphilis, chlamydia and to a lesser extent gonorrhea, I hope you also will use condoms for anal sex with new or non-monogamous partners. Gonorrhea isn't to be taken likely, and of course there are other STDs you should hope to prevent, like herpes, HPV, and Mycoplasma genitalium. 

I hope this information is helpful. Let me know if anything isn't clear.

HHH, MD